To find out how much Mrs. Perez should write the check for, we need to subtract the amount she paid in cash from the total cost of the washing machine.
The total cost of the washing machine is $986.23 and she paid $497.80 in cash.
Now, we will perform the subtraction:
\[ 986.23 - 497.80 = 488.43 \]
Therefore, Mrs. Perez should write the check for $488.43.